Scientists believe that paracatenula appeared about 500 million years ago. And all this time, the creatures have done well without a mouth and a digestive system. The point is that is that they are host to Riegeria bacteria, which excrete a full range of substances the flatworms need.
The flatworm itself is transparent, but its almost entire body is filled with white bacteria which use chemical reactions involving carbon dioxide and hydrogen sulfide to produce energy.
With evolution, the Riegeria genome has become very simple, and now they perform only the most basic functions that ensure this species’ survival. According to biologists, it is a unique example of simple symbionts producing such volume of organic substances.
It is noteworthy that the flatworms do not have the digestive function either, absorbing nutrients in droplet form. This allows Riegeria to remain intact.